(1) Field of the Invention
The invention relates generally to non-air breathing power systems and, in particular, to a closed Brayton cycle propulsion system using direct heat transfer.
(2) Description of the Prior Art
Torpedoes and other underwater vehicles use propulsion systems having turbines powered by the reaction of an oxidant with a metal fuel in a liquid state, hereinafter referred to as liquid metal fuel, as a heat source. Lithium or another alkali metal is commonly used as liquid metal fuel with sulphur hexafluoride, SF6, as the oxidant. A chlorofluorocarbon, such as C2F3Cl3 known in the art as Freon-13, can also be used as the oxidant. Another possible liquid metal fuel is an aluminum-magnesium alloy with O2 as the oxidant. Chlorofluorocarbons cannot be used with an aluminum-magnesium fuel because AlCl, one of the products of the reaction, is gaseous at operating temperatures.
Current underwater propulsion systems are typically closed Rankine cycle power systems utilizing lithium as a liquid metal fuel, a chlorofluorocarbon as an oxidant, and water as a working fluid. In a typical Rankine system, the working fluid is compressed, heated until vaporization, and then expanded through a turbine to produce power. Upon exiting the turbine, the low pressure vapor is condensed to a liquid, and the cycle is repeated. The working fluid is heated as it passes through heat transfer tubes that are wrapped to form a cylindrical annulus within the system's heat exchanger. Liquid metal fuel is contained in the center of the cylinder in order to heat the working fluid being carried by the heat transfer tubes. The working fluid, water, and the liquid metal fuel, lithium, react chemically with one another. A leak in the heat transfer tubes causes a violent reaction which generates a significant amount of heat and gas causing the heat exchanger and the underwater device to fail. Furthermore, should a leak occur in a land-based system, a toxic cloud of LiOH will be released into the environment. Other problems associated with the Rankine cycle include noise generation during the phase change of the working fluid, severe stress of the oxidant's injectors due to high reaction zone temperatures, and slow start-up time.
An alternative to the closed cycle Rankine system is the closed Brayton cycle system. In a Brayton cycle, there is no phase change and accordingly, no noise associated therewith. The Brayton cycle is also more efficient than the Rankine cycle despite the fact that more energy is required to compress a gas than to pump an equivalent mass of liquid. Prior art Brayton cycle systems cannot be used in underwater systems because the components of the closed Brayton cycle, principally the conventional Brayton heat exchanger, are too large to be used in the restricted space available in underwater vehicles.
A compact heat exchanger can be made by increasing gas velocity to achieve higher heat transfer coefficients; however, this results in greater heat exchanger pressure drop. This method is used successfully in the Rankine cycle since pump power is a small fraction of gross power ( 1/50) and pump losses are small by comparison. Accordingly, there is no significant reduction in cycle efficiency. In the Brayton cycle, however, compressor power is typically a large part of the gross power (½); therefore, small increases in gas velocity and heater pressure drop reduce the Brayton cycle efficiency below that of the Rankine cycle.